1. Field of the Invention
The present invention relates to a TTL (Through The Lens) metering device in a single lens reflex camera and more particularly relates to such type of TTL metering device which comprises a swing-up mirror adapted for a finder and at least partly having a light transmitting area therein, a light receiving system including a light-sensing element and a second mirror for directing light passed through the light transmitting area toward the light receiving system.
2. Description of the Prior Art
Between the above described type of TTL metering device and another conventional type of TTL metering device comprising a finder screen disposed before its light receiving system there is a difference in metering characteristics for exposure setting at opening aperture or at stopping down. This difference is caused by the fact that the light receiving system of the latter has a broader distribution of sensitivity than that of the former owing to the finder screen and other additional element contained in the light receiving system of the latter. This difference in metering characteristics becomes inconvenient when it is desired to use one and the same interchangeable lens in common for both of two cameras, one equipped with the first mentioned type of TTL metering device and another with the second mentioned one.
A solution to the problem is disclosed in Japanese patent application laid open No. 17,817/1977 (corresponding to DOS No. 2,627,248), which is characterized by the provision of a diffusing surface on the semitransparent portion of a swing-up mirror or on a second mirror.
While this solution may have an effect to broaden the sensitivity distribution, a large portion of effective light which otherwise would be received by the light receiving system is diffused away by the diffusing surface at the same time. Therefore, the solution has a disadvantage that the output of the light-sensing element drops down.
On the other hand, it is unallowable to enlarge the second reflecting mirror so much because it must be retracted to its second position out of the light path for taking a picture following the movement of the swing-up mirror in a manner as to cover the semitransparent portion of the swing-up mirror against light. Due to the fact, there occurs the problem of shortage of the quantity of light incident upon the light-sensing element in particular when the lens aperture is relatively large. Thus, when the lens opening is changed by one step (the value of aperture stopping is multiplied by 1/.sqroot.2), the output of the light-sensing element can not change by one step (the output of the element becomes twice larger) at the same time. In other words, the value of aperture and the output of the light-sensing element can not change proportionally in term of step. Therefore, if a picture is taken relying upon the indication made by such metering device, then an over-exposed picture will be produced. This true particularly when the lens opening is relatively large.
A known solution to this problem is to provide a signal pin on the side of lens and a correction mechanism on the side of camera body. The signal pin produces a signal informing of the value of lens aperture or the distance of exit pupil and the correction mechanism receives the signal and corrects the output of the light-sensing element accordingly. However, as will be seen easily, the provision of such additional signal pin on the lens side and such correction mechanism on the camera body side inevitably increases the manufacturing cost and also makes the structure thereof complicate.